1. Field of the invention
This invention relates to a magnetic tunnel device exploiting the magnetic tunnel effect. More particularly, it relates to a magnetic tunnel device having a granular thin film.
2. Related Art
In a three-layered structure comprised of a magnetic metal layer, an insulating layer and a magnetic metal layer, a magnetic tunnelling effect is observed if the insulating layer is tens of angstrom thick. The magnetic tunnelling effect signifies the phenomenon in which, if the voltage is applied across both magnetic metal layers, the conductance of the tunnel current flowing through both magnetic metal layers depends on the relative angle of magnetization of the magnetic metal layers.
With this magnetic tunnelling effect, the magnetic resistance ratio can be calculated from the rate of polarization of magnetization of the two magnetic metal layers. For example, an extremely high magnetic resistance ratio was estimated to be achieved with the use of Fe as a material for the magnetic metal layer.
Although this estimation was not realized for long, a resistance change ratio of approximately 18% was recently realized at ambient temperature with a three-layer structure of Fe/Al.sub.2 O.sub.3 /Fe, such that expectations are made not only of the mechanism of physical demonstration but also of application to new electromagnetic transducing elements.
In the above-mentioned three-layered structure comprised of the magnetic metal layer, insulating layer and the magnetic metal layer, basic investigations have barely been started as to formation of an insulating layer by spluttering oxidation in an oxygen atmosphere and formation of fine junctions by photolithography for producing a device. In the course of these investigations, the most serious problem has been the frequent occurrence of failure in producing resistance changes.
In the three-layered structure comprised of the magnetic metal layer, insulating layer and the magnetic metal layer, it is necessary for the tunnel current to flow between the two magnetic metal layers, such that it is necessary to reduce the film thickness of the insulating layer to a extremely small value. If the insulating layer is of an extremely small thickness, pinholes or the like defects are frequently produced in the insulating layer. If the insulation between the two magnetic metal layers is structurally destroyed, electrical leaks are generated to lead to cessation of the tunnel current.
That is, in the above-mentioned three-layered structure comprised of the magnetic metal layer, insulating layer and the magnetic metal layer, it is necessary to produce the insulating layer of a film thickness to permit the tunnel current to flow in a state free of defects, such as pinholes. If the conventional three-layer structure displaying the magnetic tunnelling effect is used as an electromagnetic transducer element, the magnetic tunnelling effect cannot be demonstrated due to occurrence of electric leakage.